Class D audio amplifiers are a well-known type of audio power amplifier which generally is recognized to provide energy efficient audio drive of a loudspeaker load by switching a pulse width modulated (PWM) or pulse density modulated (PDM) audio signal across the loudspeaker load. Class D audio amplifiers typically comprises an H-bridge driver with a pair of output terminals coupled to respective sides or terminals of the loudspeaker load to apply an oppositely phased pulse width modulated or pulse density audio signals across the loudspeaker. Several modulation schemes for pulse width modulated audio signals have been utilized in prior art class D amplifiers. In so-called AD modulation, the pulse width modulated audio signal at each output terminal or node of the H-bridge is switched between or toggles between two different levels in opposite phase. The two different levels typically correspond to the upper and lower DC power supply rails, respectively, such as the positive and negative DC supply rails.
In so-called BD modulation, the pulse width modulated audio signal across the loudspeaker load is alternatingly switched between three levels of which two levels correspond to the above-mentioned upper and lower DC power supply rails and the third level is zero level that is obtained by either pulling both terminals of the loudspeaker load to one of the DC power supply rails at the same time.
While such prior art Class D amplifiers are often considered to be highly power efficient compared to traditional non-switching audio power amplifiers such as class A, B, and AB amplifiers, these prior art Class D amplifiers never the less consume considerable amounts of idle power when the audio input signal is small or close to zero level. The idle power consumption leads to poor power efficiency at such small audio input signals with efficiency figures far below the often quoted 90-100% power efficiency of prior art Class D amplifiers. Operation at the quoted power efficiency is only obtained for very large audio input signals while operation within typical levels of audio input signals leads to much poorer power efficiency. The relatively poor power efficiency at low level audio input signals are inter alia caused by the switching losses occurring in semiconductor switches of the H-bridge and a ripple currents induced in load inductors and ripple voltages induced in load capacitors. The load inductor and load capacitor are normally inserted between each of the output terminals or nodes of the H-bridge and the loudspeaker load to provide lowpass filtering of the “raw” pulse width or density modulated audio signal. The lowpass filtering is required to suppress large amplitude switching or carrier frequency components of the pulse width or density modulated audio signal and avoid thermal damage to the loudspeaker or induce various types of intermodulation distortion.
However, load inductors and load capacitors of appropriate size for prior art class D audio amplifiers are often so large that they must be provided as external components to an integrated circuit containing other functions and circuits of the class D audio amplifier. Consequently, the load inductors and load capacitors add to the costs of complete amplification solutions or assemblies for portable and stationary entertainment and communication equipment such as TV sets computer audio, Hi-Fi stereo amplifiers etc. Likewise, the external inductors and capacitors require allocation of valuable board space for amplification solutions and present a potential reliability source.
Another problem associated with prior art Class D audio amplifiers is the generation of excessive levels of EMI noise by the carrier or switching frequency associated with the pulse width or density modulated audio signals that comprises essentially rectangular pulses with repetition frequencies in a range between 250 kHz-2 MHz. The high level of EMI noise complicates integration of these prior art class D audio amplifiers with other types of signal processing circuits such as radio-frequency transmitters/receivers etc.
Accordingly, class D amplifiers with reduced levels of EMI noise are highly desirable. Likewise, class D amplifiers with improved power efficiency, especially at low audio input signals levels, are also highly advantageous. Finally, it is desirable to decrease the size of the external load inductors and load capacitors to provide more compact, power efficient, reliable and less costly amplification solutions for consumer and other types of audio products.